When analyzing materials with a differential scanning calorimetry (DSC), only temperature rise measurements are usually taken. However, in many cases, temperature rise measurements alone are not sufficient to understand and characterize the nature and behavior of the sample. In this case, cooling measurements are an easy way to gain valuable additional information.
If you want 1Distinguish between materials with different thermal histories, e.g. between new materials and materials;2.Check for differences in molecular structure or composition;3.Rapid analysis of phase behavior of materials with mesocrystalline structures, such as liquid crystals4.Separate several overlapping thermal effects. Then the data from the cooling experiment will be extremely useful. This article is a series of experiments that illustrate the advantages of cooling measurements. These experiments were performed using a differential scanning calorimeter with a built-in cooler and automatic sample filling.
Distinguish between new and recycled polypropylene
Differentiating between new and recycled materials is an important aspect of product development and quality assurance. In many cases, there is a clear difference in the crystallization behavior between recycled and new materials.
Figure 1 shows the heating and cooling curves of new and recycled polypropylene (PP) measured at a rate of 10 kmin. It can be seen that the heating curves of the two samples are almost identical. However, the cooling curves of new and regenerated PP showed significant differences in crystallization behavior. The temperature at which the regenerated PP begins to crystallize is slightly higher. In addition, the crystallization peaks of the recycled material are wider and not as high as those of the new PP. However, the enthalpy of crystallization (peak area) is essentially the same and is independent of the type of PP.
These results show that the main difference between the two PP samples is the crystallization behavior. The reason for this is that the recycled material has a higher content of amorphous crystalline nuclei.
The cooling curves of several samples of similar weight, prepared in the same way, are very reproducible. This means that crystallization behavior can be used as a criterion to distinguish between novel and recycled PP samples.
Characterization of different polypropylene samples
Polymers are often modified by structural modification or by blending additives with polymers to optimize critical performance in specific applications. The following differential scanning calorimetry (DSC) measurement (see Figure 2) shows the secondary heating and cooling curves of three different polypropylene (PP) samples: a standard PP film, a nucleated PP, and a resin-modified PP.
The heating curves did not show any significant difference except for a small melting peak at 123°C in the resin-modified PP curve. Obviously, it is not possible to distinguish between these three materials in this way alone. In contrast, the cooling curve shows a greater difference. As expected, the nucleated PP crystallizes at higher temperatures and more rapidly than standard PP. The peak temperature is 1169°C, which is about 3K higher than standard PP. However, nucleation does not affect the enthalpy of crystallization of PP. The cooling curves of the resin-modified PP sample showed that the material crystallized at lower temperatures and indicated that the resin interfered with and hindered the crystallization process. In addition, its crystallization enthalpy (889 j g) is about 7% less than the other two samples.
Characterization of liquid crystal materials
Differential scanning calorimetry (DSC) is an excellent method to study the phase transformation of liquid crystal materials. For rapid characterization of unknown samples (for screening purposes), a ramp rate of at least 10 k per minute is typically used to save time. Under these conditions, the main melting peaks (solid-liquid) may overlap with some liquid-liquid phase transitions, so these phase transitions may go unnoticed. In this case, controlled cooling experiments are a very effective and convenient method to ensure that virtually all transitions are identified. Figure 3 shows the heating and cooling curves of the cholesteryl ester myristate, measured at a rate of 10 k per minute. The warming curve shows a major melting peak at 73°C(A) and a small peak at 83°C(C). This is due to the transition from non-isotropic liquids to isotropic liquids. At about 77°C, a shoulder (b) can be identified in the main melting peak, but it cannot be definitively interpreted as a phase transition. In the cooling experiment, except for about 20 °C (a'), two small, separated peaks appear at about 80°C (C') and 72 °C (b'）。These small peaks correspond to peak C and shoulder B. They are the peaks of the transition between the phases of the liquid crystal dielectric. In general, this type of transition is less subcooling than the actual crystallization process. In addition, the enthalpy changes associated with these transitions are also much smaller. The heating curve can be explained as follows. After melting (a), the cholesteryl ester myristate exists in a layered liquid crystal state. The subsequent acromion (B) marks the transition to the cholesterol phase, and at point C, the liquid loses its liquid crystal properties and melts to form an isotropic liquid. This example shows that overlapping transitions at elevation can be separated in cooling experiments because the subcooling behavior is different. Such experiments can be optimized by slowly heating and cooling, and by using smaller sample weights.
Glass transition overlapping with melting
Differential scanning calorimetry (DSC) cooling experiments can also be used to separate other types of overlapping effects. This is illustrated in Figure 4 using a mixture of polyethylene terephthalate (PET) and wax for the sample. As shown, the sample is first heated at a rate of 10 K per minute, then cooled at a rate of 10 K per minute, and finally heated again at a rate of 10 K per minute. The first heating process shows two overlapping endothermic peaks (a and b). These two peaks may initially be interpreted as two overlapping melting peaks. However, in the second heating process, only one endothermic peak d was shown, and a small apparent exothermic effect was shown in the original position of peak b, rather than peak b. The additional information obtained through the cooling curve makes the explanation for this behavior clear. The cooling curve shows an exothermic peak c that shifts towards low temperatures, compared to the peaks (a and d) observed in the first and second heating curves. This phenomenon is again attributed to supercooling. Moreover, the apparent step between 60 °C and 80 °C in the cooling curve clearly indicates the presence of a glass transition. Now the two heating curves can be explained as follows. During the first heating process, the wax melts (peak a);At the same time, the PET undergoes a glass transition, accompanied by an endothermic relaxation peak (Peak B). During the second heating, this relaxation peak no longer appears, and the apparent "exothermic" peak immediately following the melting peak of the wax actually corresponds to the final part of the PET glass transition. The shift of the baseline before and after the peaks (A and B, and D) also indicates that the melting peaks are overlapped by the glass transition.
The Differential Scanning Calorimeter DSC can be used to perform controlled cooling experiments to investigate effects such as glass transition, crystallization, and polymorphism to obtain information about the sample. The crystallization of the material is observed as an exothermic peak in the DSC curve. Typically, the crystallization peak occurs at a lower temperature when the corresponding melt peak heats up, because the melt is too cold. The modification of a substance usually affects the crystallization behavior more than the melting behavior. Therefore, the cooling curve is superior to the heating curve in characterizing and distinguishing similar materials. Polymorphism transitions can occur during heating and cooling. Here, since the various transitions exhibit different subcooling behaviors, effects that would otherwise overlap in the temperature rise measurement can be separated in the cooling experiments. The glass transition can be identified as a step change in the DSC curve. If the glass transition and melting peaks overlap in the temperature rise measurement, the two effects can often be separated in the cooling experiment due to crystallization shift due to melt supercooling. Overall, the warm-cool-temperature experiment provides a richer amount of information about the sample than a single temperature rise measurement. Therefore, such experiments are recommended, especially for unknown samples. The maximum controlled cooling rate of the sample depends on the temperature range and the cooling possibilities of the instrument (air cooling, intracooler, liquid nitrogen cooling). More details about this can be found in usercom 11. Microscopy experiments can be performed with a camera that is lower than a high-sensitivity CCD camera. A new accessory allows you to upgrade your HP DSC827 E to a pressure DSC-reflected light microscope system. With this system, you can perform highly sensitive DSC measurements under pressure while observing the sample. Microscopy also allows you to detect processes that produce little or no enthalpy change (e.g., color change) or to interpret effects that appear only as peaks on the DSC signal (e.g., polymorphic transition). The microscope can easily detect color changes or structural changes in the sample. You can also configure a camera and hot stage on the microscope to perform transmission microscopy experiments (e.g., FP82). With the Mettler Toledo FP84, transmission microscopy and DSC signal measurements can even be performed at the same time. The advantage of transmission microscopy is that polarized light can be used, so birefringent crystals can be clearly detected. For more detailed information on chemiluminescence and microscopy, please request the appropriate information sheet from your sales representative.